When generally manufacturing a product having an L-shaped portion, such as a front pillar reinforcement that is one of vehicle body frame parts in an automobile, by sheet-forming a metal sheet, a sheet forming tool constituted of a punch, a die, and a blank holder is used. In this case, the punch and the die are brought closer to each other in a state that the whole periphery of the metal sheet is held by the die and the blank holder to apply drawing to the metal sheet.
In such sheet forming, when sheet-forming an L-shaped part having a bent portion being bent steeply, wrinkles easily occur in a top portion of the L-shaped part. When the holding force by the die and the blank holder is increased, the wrinkles are reduced, while cracks easily occur in a shoulder portion or a side wall portion of the L-shaped part. In recent years particularly, in order to achieve the enhanced safety and the weight reduction of an automotive body, the strength of the metal sheet used as a blank has been increasingly enhanced and hence, it is impossible to expect a blank ductility equivalent to that of the conventional soft steel sheet. Accordingly, it is important to prevent cracks or wrinkles at the time of sheet forming.
As a technique of sheet-forming an L-shaped part, patent literature 1 proposes a method for draw-forming a blank shallowly first and thereafter, further bending-forming the blank by using another sheet forming tool to form the blank into a final product shape. Furthermore, as another technique with respect to a frame member having an L-shape, patent literature 2 proposes a method of sheet forming that uses a sheet forming tool unit constituted of a die, a pad, and a bending tool.